Natural leather has, until recently, been the primary material used in the production of shoe uppers. While leather has many properties which are desirable for shoe uppers, certain disadvantages thereof have caused difficulties in the production of shoes. For example, quality variation between hides and defects and differences within a hide vary the quality of shoe uppers produced from hides. Additionally, quality control of leather is difficult to achieve, and this renders somewhat unpredictable the quality of individual hides or a portion thereof. Considerable wastage also occurs in making shoes due to the irregular shape of hides.
For these and other reasons, considerable efforts have been made in the art to produce man-made leather substitutes for the production of shoe uppers. These substitute materials are commonly referred to, collectively, as artificial leathers, although this term is not completely accurate in that the substitute materials vary widely in their properties and some of the substitute materials have few properties in common with leather. In one respect, these differences between the properties of leather and substitute materials are the results of efforts to provide properties for shoe uppers which are not provided by leather, and, in another respect, these differences are the results of the inability of the art to reproduce certain of the desirable properties of leather.
Natural leather is, essentially, a fibrous material having bundles of fibrils which overlap and entwine to form a porous material. The fibrous structure of leather provides the desirable properties of flexure, suppleness, relatively high modulus, tear resistance and good tensile strength, which properties are important for providing a very wearable and durable material. Although leather is somewhat abrasion resistant, it is susceptible to scuffing on the surface and requires relatively frequent polishing to maintain a pleasing appearance. While the macroporous nature of the leather provides relatively high moisture transmission rates and liquid moisture absorption capacity, it also allows liquid water to penetrate through the leather, which requires the wearer of leather shoes to avoid getting the shoes wet.
In the efforts by the art to provide artificial leathers, considerable attention has been given to improving scuff resistance and to reduce liquid water transmission, thus obviating some of the disadvantages of leather in shoe uppers. Artificial leathers are, generally, compositions which contain substantial proportions of synthetic polymers, e.g., polyurethane or polyvinyl chloride, usually as a film on the outer surface of the artificial leather, and these compositions usually provide a relatively tough, water impermeable and chemical resistant polymeric outer surface. Artificial leathers of this nature have met with some success, but the consuming public has not accepted these artificial leathers as a general substitute for leather due to other undesirable properties thereof. Low cracking resistance with continued flexing of the artificial leather has presented a significant problem, and, in some cases of artificial leather, cracks will appear with only several months of wear. Also, while the artificial leather can present good scuff resistance, the material is often subject to peeling of the outer polymeric film as a result of relatively deep scuffs or cuts. This peeling cannot be effectively repaired by polishing, as is the case with leather. The consuming public has also been well aware of the relatively low moisture transmissions from the inside of the shoe made with artificial leather uppers, which results in an uncomfortable dampness on the feet of the wearer. This difficulty is accentuated with increased thicknesses of the outer polymeric film and with the total content of polymeric materials in the artificial leather composition.
While the above disadvantages are quite serious and have reduced the acceptance of artificial leathers, several other disadvantages have been of overriding difficulty and have severely restricted the acceptability of artificial leathers for a large segment of the consuming public. Films or layers of synthetic polymers, alone, have serious disadvantages in that they do not resist tearing, particularly when portions of a shoe upper are stitched together. Also, when the polymeric materials are sufficiently soft to provide an acceptable hand and feel, they are inherently so flexible and of so low modulus that they stretch and have little shape-retention ability (stability). Thus, it has been generally necessary to provide a composite of a polymeric film material and a reinforcing backing, which reinforcing backing gives extended wear to the shoe upper and provides shape retention properties. Additionally, the reinforcing backing is required to provide sufficient tensile strength of the material so that it is capable of being stretched over a shoe last without tearing or other related damage. Reinforcing backings are, generally, either a nonwoven textile or woven or knitted textiles or a combination thereof. A woven or knitted textile is sometimes positioned between the polymeric film and a nonwoven textile to make up a conventional artificial leather. Such a nonwoven textile is typically made by needling a mat of fibers and then splitting the needled fibers in planes parallel to the running length thereof to produce a plurality of "wafers" each having a thickness of about one thirty-second to one-sixteenth inch.
The reinforcing backing material used with these artificial leathers, while necessary for the above reasons, has resulted in a material which is not yieldable in the manner of leather. The fibrous nature of leather allows the fibers to slip, relative to one another, under continued stress and thereby take a new permanent configuration, which is known as "set" in the art. The consuming public experiences this phenomena in the wearing of shoes and generally refers to the phenomena as "breaking in a new pair of shoes". As a new pair of leather shoes is worn, individual differences in foot shape apply stress to the leather uppers, and the fibers of the leather under this stress slowly slip relative to one another and take a set which conforms to the individual shape of the foot. After only a reasonable amount of wear, the leather has, in effect, been custom molded to the foot of the particular wearer and provides exceptional comfort to the wearer. In addition, leather will take a set up to a maximum elongation or stretch. After that maximum stretch, the leather will further stretch only under considerable stress. Unfortunately, conventional artificial leathers with the relatively large proportions of polymer film and woven reinforcing backing have not provided these set properties of leather.
While some elasticity is provided in conventional artificial leathers, this elasticity does not result in a permanent set in the nature of leather. Conventional artificial leathers may be extended according to the individual shape of the foot during wear on any one day, but after the shoe is removed from the foot for a significant length of time, e.g., overnight, the artificial leathers return, essentially, to the shape originally provided in the manufacture of the shoe. This characteristic is known in the art as the "memory" of the artificial leather. Thus, the wearer is required to "break in" the shoes each day that the wearer uses the shoes, at least until sufficient wear during the day has been accomplished to once more extend the artificial leather uppers to conform to the particular shape of the individual foot. The degree of "memory" varies somewhat with various conventional artificial leathers, but all of the materials conventionally used suffer from the effects of memory. The effects of memory are particularly acute with thick polymer films and with backings which use a textile woven of tightly spun yarns. When the backing is in part a woven product, the strength of the artificial leather is greatly improved, but the woven textile in the backing makes the memory effect extraordinarily serious. Accordingly, discomfort is experienced by the wearer of conventionally backed artificial leather at least for part of each day that the wearer uses the shoes made thereof.
The degree of the discomfort, noted above, depends upon how closely the feet of the wearer approximates the shape of the particular shoes. If the wearer's feet closely approximate the shape of the shoes, then this discomfort is minimized, but if the wearer's feet do not closely approximate the shape of the shoes, then this discomfort can be totally unacceptable. Thus, a proper fit of the shoes made with artificial leather is necessary in order that the shape of the shoes approximate the shape of the feet of the wearer as closely as possible. However, the accuracy of the fit, when purchasing shoes, is complicated by the degree of care exercised by individual shoe salesmen and by the practice in the art of limiting the number of sizes and widths of shoes to a relatively small number. Thus, if a proper fit for an individual foot is not provided by a standard size or width and the buyer accepts a somewhat improper fit, then the discomfort noted above is accentuated.
In addition to the foregoing, the last used in forming shoes varies somewhat with the particular manufacturer and with the style of shoes. Thus, for example, a size 9-D in one style or from one manufacturer may fit a particular wearer whereas a 9-D in another style or from another manufacturer may not. While these differences, generally, are overcome by "breaking in" leather shoes, these differences can be critical with shoes made of artificial leather, and the confusion to the buyer in selecting a proper size accentuates the problem of obtaining the correct fit required for shoes of artificial leather. All of the foregoing is additionally complicated by the fact that the shape of feet changes during the day and during seasons, which can render a proper fit selected during the morning or in the winter an improper fit during the evening or in the summer.
A further serious problem of artificial leathers is the position of axis of flexure. The axis of flexure of leather is essentially parallel to and displaced toward the upper or face surface of the leather, as opposed to the center or the back surface of the leather. With the axis of flexure being near the face surface of the leather, as opposed to, particularly, the back surface of the leather, leather may bend with only relatively low face surface strains and is, therefore, able to withstand flex cracking of the face surface. Additionally, in walking, leather bends -- grain inwardly -- in the vamp of a shoe and the radius of curvature of the bent leather results in wrinkles on the face surface of the leather, commonly called the "break". A coarse "break" makes an unsightly appearance on the face surface while a fine "break" provides a pleasing appearance. A fine "break", e.g., up to about 20 wrinkles per inch, results from the flex axis being near the surface and connotes good quality leather. Conversely, a coarse "break", e.g., as low as six wrinkles per inch, is characteristic of "split" leather with a flex axis near the center of the material and indicates poor quality leather. Observing the "break" in leather has been a traditional customer test for good quality leather. Artificial leathers without a highly woven fabric near the face surface have not provided an axis of flexure sufficiently close to the face surface to provide a pleasingly fine "break" in the nature of quality leather.
Closely akin to break, is the folding in the vamp of a shoe, and the folding is related to the position of the flex axis of the material and the bulk density of the material of the vamp. As a wearer walks forward, the folds begin to form, and at the instant before the foot leaves the ground, the folds are at a maximum curvature. If the folds have large radii of curvature, then the amount of material in the folds becomes large and effectively reduces the amount of material forming the wearing length of the vamp, which correspondingly decreases the internal volume of the shoe. The wearer experiences this action as a tightening or pinching of the foot when the fold curvatures are excessive. Thus, in many artificial leathers, the radius of curvature of the folds have not been in an acceptable range. In some shoes of artificial leather, the break and fold in the vamp will be quite excessive and make the shoes both unsightly and uncomfortable.
One form of artificial leather has been made by laminating a relatively high modulus structure, e.g., a woven or knitted fabric, between a thick polymeric coating and a reinforcing backing. This approach, inherently, will somewhat shift the axis of flexure toward the face surface of the artificial leather, but the high modulus woven or knitted structure accentuates the problem of set, noted above, and correspondingly increases the discomfort experienced each day when the shoe is worn.
Artificial leathers also have suffered from very serious disadvantages in the manufacture of shoes. When shoes are made of leather, the cut leather (sometimes wet) is stretched over the last in the forward direction toward the toe of the shoe. This stretching causes a contraction of the leather in the widthwise direction (across the vamp) and allows the leather to tightly conform to the shape of the last without bulges or wrinkles across the vamp. In effect, the leather has a relatively constant surface area under stress, i.e., if elongated in one direction it correspondingly contracts in the transverse direction and the elongated area is about the same as the area before elongation. Artificial leathers have not provided this property of relatively constant area under stress and it has been necessary to significantly complicate the traditional shoe making process with additional and difficult processing steps. Even with special patterns and special processing, it has not been possible to make certain styles and some shoes when removed from the shoe forming lasts loose their intended shape. These difficulties have severly limited the economic use of and number of styles produced with artificial leather.
As can be easily appreciated from the foregoing, artificial leathers suffer from numerous disadvantages, as opposed to leather, which have severely restricted the use of artificial leather. Most of the disadvantages are results of the inability in the art to provide artificial leathers with certain of the properties of leather. It would, therefore, be desirable to provide an artificial leather which provides such certain desirable properties of leather and which also provides the desirable advantages of conventional artificial leather.